The invention relates to a device for optically coupling a radiation source unit to at least one optical transmission fiber. The device comprises a radiation source housing provided with a lens system and a transparent window whose outer side is connected to the transmission fiber.
Such a device is described in U.S. Pat. No. 4,355,323. In the known device the radiation emitting surface of a single radiation source, which may be a light emitting diode (LED) or a semiconductor laser, is imaged on the entrance aperture of a transmission fiber by a spherical lens. The lens is arranged on the inner side of the window. The transmission fiber is a multimode fiber in which a plurality of radiation modes can propagate simultaneously.
As is stated in an article by D. Marcuse entitled "Loss Analysis of Single-Mode Fiber Splices" (The Bell System Technical Journal, Vol. 56, No. 5 , pages 703-718 (May-June 1977)), the radiation field of a monomode transmission fiber bears much resemblance to a Gaussian intensity distribution. Thus, a model spot may be assigned to such a fiber, which spot is characterized by a spot diameter 2W. W is the distance between the point of maximum intensity in the spot and the point where the intensity has decreased to 1/e.sub.2 of the maximum value.
The modal spot diameter 2W is the principal parameter in the case of coupling to a monomode transmission fiber. A light beam is almost wholly accepted by such a fiber only if the beam spot diameter is also 2W at the location of the end face of the fiber. For a monomode transmission fiber the modal spot diameter is on the order of the core diameter, and is, consequently, much smaller than the core diameter of a multimode transmission fiber. The principal coupling parameters for a multimode of fiber are the core diameter and the numerical aperture.
When a monomode transmission fiber is coupled to a radiation source, the source should be imaged as a very small radiation spot. This means that stringent requirements must be imposed on the optical path lengths between the radiation source and the end face of the fiber. Moreover, since the modal spot and the source spot are so small, these spots should be aligned very accurately relative to one another. Therefore, stringent requirements must be imposed on the alignment of the optical components, such as the radiation source, the lens system and the transmission fiber. A coupling device which meets such stringent requirements is very susceptible to temperature and external mechanical influences.